Photoconductive device having a silicon dioxide protective layer and method of making same



March 24, 1959 FLOYD, JR 2,879,360

PHOTOCONDUCTIVE DEVICE HAVING A SILICON DIOXIDE PROTECTIVE LAYER ANDMETHOD OF MAKING I Filed 001". l, 1956 ,Q'CEY A. AZOVD INVENTOR.

PQLZEIILOM United States Patent CON DIOXIDE PROTECTIVE LAYER AND -METHODOF MAKING 'SAME Acey L. Floyd, Jr., Duarte, Calif., assignortoLane-Wells Company, Los Angeles, Calif., a corporation of Cali for-maApplication October 1, 1956, Serial No. 613,173 11 Claims. (Cl. 201-63)This invention relates to photoconductive devices; and more particularlyto improvements in such cells and improved procedures for producing andprotecting them.

It is known to prepare thin-film type photoconductive cells by vacuumsublimation of CdS and receiving a deposit of the sublimation .productupon a suitable base, the deposit being formed between or havingthereafter applied, suitable electrodes. Several objectionable featurescharacterize cells thus produced. In general, they lack uniformity ofelectro-optical characteristics such as sensitivity, are non-ohmicexcept in rare and unpredictable instances, and some cells fail toattain any appreciable degree of photoconductivity. In aconcurrently-filed application of applicant, Serial No. 612,929, thereis disclosed a procedure wherein a vacuum deposited photoconductive.film is subjected to heat treatment in excess of that required toproduce maximum sensitivity, whereby the film is rendered substantiallyinsensitive. The surface portion of the film 'is removed, leaving a filmhaving a high degree of sensitivity. Apparently during those stages ofthe heat treatment during which maximum sensitivity is approached andimmediately thereafter, a poisoning surface layer is produced on the CdSfilm, which causes the sensitivity to decrease to substantially zerovalue. Removal of the surface layer of the film leaves only the highlysensitive remainder of'the film in place on .the base.

The present invention has for a principal object, provision of aprocedure for producing a photoconductive microcrystalline film of veryhigh sensitivity without danget of producing the aforementionedpoisoning surface anomaly on the film; that is, for producing, withcertainty and with only a small number of steps, highly sensitivephotoconductive films. Another object of the invention is to provide animproved type of photoconductive cell of the thin-film variety. Anadditional object is to provide a photoconductive film or layer, havinga protective covering. An additional object of the invention is toprovide a procedure for protecting photoconductive cells of the naturementioned. Another object of the invention is to provide a procedure forproducing photoconductive cells of uniformly excellent characteristics.The mentioned objects and other objects and advantages are attained bythe invention, as will be made apparent or as will be evident followingconsideration of the description of an exemplary procedure according tothe principles of-the invention. The novel procedures are hereinafterdescribed in connection with the accompanying drawings illustrating insomewhat diagrammatic form an exemplary cell of the category mentioned,and in which drawings:

Figure 1 is a view of an exemplary cell base, with electrodes andelectrical leads in place;

Figure 2 is a view of the unit shown in Figure 1, with a surficial layerof photoconductive cadmium sulphide or the like applied; and

Figure 3 is a view of the cell unit with a protective coating appliedover an essential area of the photoconductive material.

In the drawings, designates a cell base, which in the selected form ofthe cell chosen for purposes of illustration, is a rectangular block ofelectrically insulative material such as glass. It is to be noted thatthe cell 2,879,360 Patented Mar. 24, 1959 base may be'ofotherconfiguration, and of other suitable material. Suitably applied tobase 10 by any appropriate procedure, as forexample, that described inapplicants copending application Serial No. 489,397., filed February 21,1955, is an array or arrangement of electrically conductive thin-filmelectrodes 12, 12, of chromium, gold, indium or other suitable metallicmaterial. Preferably, and in accord with considerations disclosed inconcurrently filed applications of Czipott and Floyd, Serial Nos.613,171 and 613,172, the electrodes 12 are formed of indium. Suitableelectrical leads 14 may be soldered or otherwise afiixed in conductingrelationship with respective electrodes 12.

In accord with the novel procedures provided by the invention, a.surficial film or layer of a photoconductor material 16 is applied orformed over and between electrodes 12. This layer is designated bystippling in Figures 2 and 3, and is of material that is photoconductiveor potentially photoconductive; such as cadmium sulphide. The layer maybe applied by sublimation of the material 16 in vacuo and concurrentlythere receiving a deposit of the sublimation product on the base andelectrodes. This mode of applying a thin layer or film of material ontoa surface is well known as vacuum evaporation, and may be such as isdescribed in the aforementioned patent application of Floyd, Serial No.489,397. Material 16 may be to some extent photosensitive as depositedin the vacuum chamber, but whether sensitive or not, is preferably heattreated or baked sutiiciently to transform the groups of particles ofmaterial into a form in which a much higher degree of photosensitivityis exhibited. For example, the cell unit including material 16 may besubjected to a temperature of 400 C.- 500 C. for a periodof sufiicientduration to insure complete sensitization of the material. This heattreatment or baking of the photoconductor layer may alternatively eitherprecede, or follow, application of protective material over material 16.Preferably the photoconductor (which may include so called impurities)is baked or otherwise heated to a state of high photosensitivitysubsequent to application of the protective material.

Application of material adapted to form or Provide a protective cover orcoating over all or a desired area of the photoconductor layer may beeffected according to any suitable procedure, but preferably isperformed by vacuum sublimation and condensation of suitable covermaterial in a manner similar to that by which the photoconductor layeris laid down on the cell base. A preferred basic material for theprotective layer or film is silicon monoxide, since that materialsublimes at a relatively low temperature (somewhat in excess of 200 C.and may, by suitable heat treatment in the presence of oxygen, beconverted into silicon dioxide to form a substantially impervioustransparent cover. The surficial layer or deposit of silicon monoxidethus applied to the cell unit by vacuum sublimation or otherwise may beas thick as is desired or required; and since the final protective filmwill be substantially pure quartz, it will not adversely, affect theelectro-optical characteristics of the cell.

As previously indicated, the cell unit with the deposited layer or filmof silicon monoxide is preferably next subjected in the presence ofoxygen to heat of suflicient degree and intensity to convert the siliconmonoxide to silicon dioxide. This heating may, for example, be at atemperature somewhat above 200 C. and is continued until the indicatedconversion has been substantially or wholly completed. Thereafter thephotoconductor film may be subjected to the sensitizing heat treatmentif the latter was not performed prior to deposition of the siliconmonoxide layer. In Figure 3 of the draw ings a completed cell isdepicted, with the-protective only an essential portion of material 16,that is, the portion disposed between conductive electrodes 12.

Thus a highly sensitivephotoconductive cell is provided, which has allor any essential area or portion of the photoconductive material fullyprotected by a film of fused silicon dioxide intimately bonded to thephotoconductive material. The protective cover is substantiallycontinuous and substantially impervious, so that substances which couldotherwise poison the cell are ex cluded, and the cell surface is alsomechanically protected. Further, the protective film may be so thin andso optically transparent as to have substantially no adverse efl ectupon the cell. Additionally, the protection afforded is permanent,whereby excess oxygen or other deteriorating substances are prohibitedfrom gradually entering and degrading the photoconductive material.While by the described exemplary procedure of depositing cover materialon the cell base by subliming in vacuo provides a substantiallycontinuous deposit over all of the exposed surface, it is evident thatonly an essential, or a predetermined areal extent, of the cell surfaceneed be covered. Any area upon which the cover is not required ordesired may be masked during deposition of the silicon monoxide. Theparticular material used to form the photoconductive layer is not ofprimary importance in the present invention, it being essential onlythat that material be actually or potentially photosensitive. In theexemplary cell according to the invention it is preferable that cadmiumsulphide form a principal fraction of the photosensitive material, andaccordingly a material of the composition specifically disclosed in aconcurrently filed patent application of Akos Z. Czipott and Acey L.Floyd, Serial No. 613,171, may be employed. Y

Since in the light of the present disclosure, modifications of thespecific exemplary procedures and materials will be suggested to thoseskilled in the art, it is not de: sired to be restricted to theparticular illustrative examples described, but what is claimed is:

l. A method of providing a protective translucent cover for a thin filmof photoconductor material, comprising depositing a thin layer ofsilicon monoxide over the photoconductive material, and heating thesilicon monoxide in the presence of oxygen to convert the siliconmonoxide into a substantially continuous thin cover of silicon dioxide.I I

2. A method of producing a thin translucent protective coating over asurficial layer of photoconductive material, comprising subliming invacuo a material consisting essentially of silicon monoxide and therereceiving a layer of the sublimed silicon monoxide on and over the layerof photoconductive material, and thereafter convetting the layer ofsilicon monoxide to a substantially continuous coating of silicondioxide by heating the layer of silicon monoxide in the presence ofoxygen.

3. A method of producing a photoconductive cell of the cadmium sulphidetype, comprising subliming onto a base in vacuo a deposit of materialcomprising principally cadmium sulphide, subliming onto said base oversaid deposit in vacuo a layer of silicon monoxide, and heat treating thecover layer of silicon monoxide in the presence of oxygen tothereby forma thin protective cover of silicon dioxide over said deposit ofmaterial.

4. A method of producing a photoconductive cell, com: prising preparingan electrically insulative base, subliming in vacuo a photoconductormaterial and concurrently there receiving on the prepared base at leasta portion of the sublimation poduct to provide on the base a thindeposit of the sublimed photoconductor material, subliming siliconmonoxide in vacuo and concurrently there receiving over and on at leasta portion of the base and coating of silicon monoxide to a coating ofsilicon diox-- ide by heating the coating in-the presence of oxygen.

5. A method of producing a surface-protected layer of photoconductivematerial, comprising forming a layer of photoconductive material,subliming silicon monoxide in vacuoand concurrently there receiving on asurface of the layer of photoconductive material a surficial 'deposit ofthe sublmied silicon monoxide, and thereafter converting the-surficialdeposit of silicon-monoxide to a substantially continuous film ofsilicon dioxide by1heating the surficial deposit in the presence ofoxygen, to provide a protective film of silicon dioxide over the layer9? of photoconductive material.

6. A method of producing a photoconductive cell, comprising preparing acell base, subliming in vacuo a potentially photoconductive material andconcurrently there receiving upon the cell base a thin layer of thesublimed material, heating the'thin layer ofsublirned'" material toincrease its photoconductivity, subliming in vacuo a mass of siliconmonoxide and concurrently there receiving upon the thin layer ofsublimed material a thin deposit of sublimed silicon monoxide, andheating the thindeposit of silicon monoxide in.the presence of oxygentive material.

of: silicon monoxide, and thereafter converting the thin to an extentsuflicient to convert the silicon monoxide tof siliconidioxide and forma thin translucent protective film overlying the photoconductivematerial.

7. A photoconductive cell comprising a base, a layer 1 ofphotoconductive material on said base, and a'thin film of silicondioxide superposed upon and protecting said' layer of photoconductivematerial.

8. A photoconductive cellcomprising an electrically insulative base, athin film of photoconductive material consisting principallyofcadmiumsulphide on said base, and a thin substantially continuous film ofsilicon dioxide covering at least an essential portion of said filmof-phot'oconductive material.

, 9. A photoconductive cadmium sulphide cell comprising a base, a thinfilm of photoconductive material consisting principally of cadmiumsulphide on said base, a thin film of fused silicon dioxide covering andin intimate contact with at-least an essential portion of said film ofphotoconductive material, and electrode means providing electricalaccess to selected portions of said film of photoconductive material.

10. A photoconductive cell comprising means providing an electricallyinsulative base surface, an array of at least two electricallyconductive means disposed in spaced-- part fixed positions on said baseand insulated each from another, a thin film of photoconductive materialextending between and photocondu'ctively' interconnecting at least twoof said array of electrically conductive means, I, and a layer ofsilicon dioxide covering and fused toat', film of photoconleast anessential portion of said thin ductive material.

11. A photoconductive cell of the cadmium-sulphide type comprising anelectrically insulative base, electrical,

terminals secured on said base, at least two discrete metallicelectrically conductive films on said baseand each electricallyconnected to a respective one of said, electric terminals, a layer ofphotoconductive material. consisting essentially of cadmium sulphidedisposed over anarea of said base and photoconductively intercon-l,

meeting at least two ofsaid electrically conductive films,

and a translucent impervious protective layer of fused.

silicon dioxide disposed over and intimately bonded to at leastanessential References Cited in the file of this patent I V UNITEDSTATES PATENTS portion of said layer'of photoconduc Gans Sept. 8 3.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Nob2,879,360 a. March 24, 1959 Acey Lo Floyd, Jr,

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorreoted below,

In the grant, lines 2. and 3, for 'ass:1'.gnor to Lane-Wells Company, of

. Los Angeles, California, a corporation of California, read assignor,

by means assignments, to Dresser Industries, Inc of Dallas, Texas, acorporation of Delaware,--; line 12, for Lane-Wells Company," readDresser Industries, Inc: in the heading to the printed specification,lines 5, 6 and '7, for "assignor to Lane-Wells Company, Los Angeles,Califn a corporation of California! read assignor, by mesne assignments,to Dresser Industries, Inc, Dallas, Texas, a corporation of Delawarecolumn. 3, line 70, for "poduct" read product Signed and sealed this28th day of July 1959.

Attest:

K ROBERT c. WATSON Attesting Officer Conmissioner of Patents UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,879,360March 24, 1959 Acey L, Floyd, J'r, It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below,

In the grant, lines 2. and 3, for "assignor to Lane-Wells Company, of

. Los Angeles, California, a corporation of California," read ass'ignor,

by mesne' assignments, to Dresser Industries, Inc. of Dallas, Texas, acorporation of DelaWare',=-; line 12, for "Lane-Wells Company, readDre'sser lndustries, Inc in the heading to the printed specification,lines 5, 6 and '7, for "assignor to Lane-Wells Company, Los Angeles,Calif,

a corporation of California! read assignor, by mesne assignments, toDresser Industries, Inc, Dallas, Texas, a corporation of Delawarecolumn. 3, line '70, for "poduct' read product Signed and sealed this28th day of July 1959.

a; SEAL) Attest:

KARL MINE ROBERT c. WATSON Attesting Officer Conmissioner of Patents

1. A METHOD OF PROVIDING A PROTECTIVE TRANSLUCENT COVER FOR A THIN FILM OF PHOTOCONDUCTOR MATERIAL, COMPRISING DEPOSITING A THIN LAYER OF SILICON MONOXIDE OVER THE PHOTOCONDUCTIVE MATERIAL, AND HEATING THE SILICON MONOXIDE IN THE PRESENCE OF OXYGEN TO CONVERT THE SILICON MONOXIDE INTO A SUBSTANTIALLY CONTINUOUS THIN COVER OF SILICON DIOXIDE. 